The present invention relates to an improvement in a hydraulic reaction force apparatus in a power steering device, for obtaining a proper steering force corresponding to various running conditions of a vehicle, such as a vehicle speed and a steering angle.
In a power steering device for reducing a steering wheel operating force (steering force) of a vehicle, an operation of the device must be properly controlled in accordance with various running conditions of a vehicle, such as a steering force and a steering angle corresponding to a steering operation of a driver, and a vehicle speed, thereby obtaining an auxiliary steering force. That is, a large auxiliary steering force must be output to require an extremely small steering force of the operator during the steering operation performed when a vehicle is parked or running at a low speed. On the other hand, when a vehicle is running at a high speed, such a large auxiliary steering force reduces the steering force too much. Then, the driver feels uneasy, and hence this is not preferable to allow safe and comfortable driving. Therefore, in this case, the auxiliary steering force must be reduced to make a steering wheel heavier, i.e., to increase the steering force, thereby obtaining stability during straight running of a vehicle. Such steering force control is similarly required as a steering angle is increased.
In order to satisfy the above requirement, a number of hydraulic reaction force apparatuses having various arrangements have been conventionally proposed. In these apparatuses, a proper rigidity (steering reaction force) is given to a steering wheel by a reaction force hydraulic pressure controlled in accordance with various running conditions of a vehicle, thereby controlling a steering force when a vehicle runs at high and low speeds and under various other conditions. More specifically, in a hydraulic reaction force apparatus of this type disclosed in Japanese Patent Prepublication (Kokai) No. 49-102092 and Japanese Utility Model Prepublication (Kokai) No. 60-169068, reaction force arms project at the input shaft side of a rotary control valve for switching flow paths, a pair of reaction force plungers are provided at the output shaft side thereof for each arm to move forward and backward from both sides along a rotation direction of the arm, and a hydraulic reaction force chamber is formed at the outer ends of the plungers. A reaction force hydraulic pressure is applied into the hydraulic reaction force chamber to drive the plungers in accordance with a vehicle speed, and a predetermined restriction force is applied on the input shaft through the arms, thereby obtaining a proper steering reaction force to facilitate the steering operation corresponding to various running conditions of the vehicle.
In the hydraulic reaction force apparatus having the above arrangement, the reaction plungers are slidably held in guide holes formed at the output shaft side, and the plungers are moved in a desired direction by a differential pressure before and after themselves (a pressure difference between the arms and the hydraulic reaction force chamber). The outer ends of the plungers strike against retainers provided as stoppers at the outer ends of the guide holes, and the inner ends thereof strike against locking step portions at the output shaft side facing the inner ends of the guide holes or the reaction force arms at the input shaft side. In this case, an impact sound is generated between metal members, thus posing a noise problem.
Especially, the impact sound generated when the plungers strike against the retainers as stoppers is generated because a return pressure is abruptly increased due to an impulsive input such as kickback from a tire, and hence is a large sound. Therefore, a countermeasure is required to absorb or reduce the above impact sound. It is a matter of course that such a problem is not posed in a hydraulic reaction force apparatus of a type in which the plungers are biased toward the arms by springs, since the impact sound is absorbed by the biasing force. However, the plungers strike against the arms or the locking step portions by the spring reaction force to generate a large impact sound. Therefore, such a problem must be taken into consideration again in this apparatus.
In order to prevent the impact sound generated between the plungers and retainers, a buffer member such as rubber may be adhered on a contacting portion of each retainer opposing the corresponding plunger. However, in this arrangement, durability or a proper absorbing effect of kinetic energy with respect to the repeatedly applied impulsive force poses another problem. In addition, a problem of an impact sound generated by the arms or the locking step portions at the opposite side remains unsolved. Therefore, some countermeasures must be taken in consideration of the above situation.